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A polyoxotungstate-surfactant hybrid layered compound was synthesized as a single phase by using decatungstate ([W10O32]4−, W10) and hexadecylpyridinium (C16py). The X-ray structure analysis combined with infrared spectroscopy and elemental analysis revealed the formula to be (C16py)4[W10O32] (C16py-W10). The layered structure consisted of alternative stacking of W10 inorganic monolayers and interdigitated C16py bilayers with layered periodicity of 23.3 Å. Each W10 anion in the W10 inorganic monolayers was isolated by the hydrophilic heads of C16py. The hybrid crystals of C16py-W10 decomposed at around 500 K. The conductivity of the hybrid layered crystal was estimated to be 4.8 × 10−6 S cm−1 at 423 K by alternating current (AC) impedance spectroscopy.

Inorganic-organic hybrid layered compounds exhibit higher structural flexibility than purely inorganic compounds owing to their organic components [1,2,3,4,5,6,7,8]. The synergy of inorganic and organic character is beneficial for synthesizing functional materials. Conductive hybrid compounds composed of organic cations and inorganic anions have been reported, and the molecular structures and arrangements of components have been precisely controlled for the emergence of conductive functions [1,2,3,4,5,6].

Combination of polyoxometalates (POMs) and surfactant cations [9,10,11] leads to promising functional inorganic-organic layered hybrids. POMs can add various physicochemical properties to the hybrids as inorganic components [12,13,14,15,16], while surfactants enable the control of layered structures as structure-directing organic components [17,18,19]. While there are many types of POM-surfactant hybrid [9,10,11,20,21,22,23,24,25], POM-surfactant single crystals are rare [26,27,28,29,30,31,32]. These POM-surfactant hybrids can allow fine tuning of the structures and functions, and are promising conducting materials as precedented inorganic-organic hybrid conductors [2,3]. In addition, polyoxotungstate-surfactant hybrids are less reducible, and more structurally stable than those of polyoxomolybdates [12,16]. Among several polyoxotungstates, a decatungstate (W10O324−, W10) works as a characteristic electron acceptor [33,34,35]. The conductivity of the hybrid crystals is expected to be enhanced by the π-electrons of surfactants such as hexadecylpyridinium ([C5H5N(C16H33)]+, C16py). However, there is no polyoxotungstate-surfactant hybrid analyzed by single crystal X-ray crystallography, while a few polyoxotungstate crystals containing a long methylene moiety (~6 methylene groups) have been reported [36,37,38].

Here we report the synthesis of a hybrid compound of C16py and W10 (C16py-W10). The crystal structure was successfully determined by the X-ray structure analysis. The conductive property was also investigated.

2. Results and Discussion2.1. Crystal Structure of C16py-W10

IR spectra of as-prepared precipitates, recrystallized samples, and crystals prepared with Na-W10 showed the presence of C16py cations and W10 anions. The single crystal X-ray structure analysis combined with the elemental analysis revealed the formula of C16py-W10 to be [C5H5N(C16H33)]4[W10O32] (Table 1). Four C16py cations (1+ charge) were associated with one W10 anion (4− charge), and C16py-W10 did not contain any H+. Figure 1 shows the crystal structure of C16py-W10. The crystal packing consisted of alternating W10 inorganic layers and C16py organic layers with periodicity of 23.3 Å (Figure 1). The W10 anions formed monolayers, while the hexadecyl chains of C16py interdigitated to form a bilayer structure, which is a typical structure for most POM-surfactant hybrid crystals [26,27,28,29,30,31,32]. All C–C bonds in the hexadecyl chains showed anti conformation except one C–C bond (C7–C8). While both C16py-W10 and C16pyCl·H2O [39] contained interdigitated bilayers of C16py with the pyridine ring inserted into the hydrophilic layers, the packing of hydrophilic layers was different; the W10 monolayers for C16py-W10 and the Cl––H2O bilayers for C16pyCl·H2O.

The hydrophilic heads of C16py penetrated into the W10 inorganic monolayers and isolated each W10 anion (Figure 2) in a similar way to that in the crystal of C16py-hexamolybdate (C16py-Mo6) [31] or C16py-α-octamolybdate (C16py-α-Mo8) [32]. On the other hand, two independent C16py cations were not parallel (angle: 52.6°) without π–π stacking, different from C16py-Mo6 [31] and C16py-α-Mo8 [32]. The C–H···π(centroid) distance was 3.32 Ǻ, and the nearest C–H bond (C24–H24) was not directed to the center of the pyridine ring. The shortest interatomic distance (C3···H24, 2.88 Ǻ) between the pyridine rings was almost the same as the sum of the van der Waals radii (2.90 Ǻ). Therefore, C–H···π interaction [40,41] was hardly observed in the present C16py-W10.

Figure 2

Molecular arrangements in the inorganic layers of C16py-W10. The hexadecyl groups are omitted for clarity.

C16py-W10 had two-dimensionally confined monolayers of the W10 anions (Figure 2). The distance between the nearest W10 anions was 3.23 Å, much shorter than that for W10 crystals composed of n-butyl- [42,43] or n-propylammonium [44] (5.4–5.8 Å). The close distance between W10 would contribute to the emergence of conductivity for C16py-W10.

C16py-W10 had C–H···O hydrogen bonds [40] at the interface between the W10 and C16py layers. The C···O distances were 3.28–3.88 Å (Table 2). The mean value was 3.51 Å, and was shorter than the mean C···O distances (~3.6 Å) in other POM hybrid crystals containing the C16py cation [31,32]. These shorter hydrogen bonds as well as electrostatic interactions between C16py and W10 would stabilize the layered crystal structure of C16py-W10 with rigid packing. Most hydrogen bonds were formed between oxygen atoms of W10 and the hydrophilic head of C16py (i.e., pyridine rings or methylene groups near nitrogen).

Powder XRD patterns of C16py-W10 were measured at room temperature (Figure 3). The XRD pattern of as-prepared C16py-W10 exhibited weak and broad peaks (Figure 3a). The XRD pattern of recrystallized C16py-W10 showed much sharper and stronger peaks (Figure 3b), while the peak positions were close to those of the as-prepared C16py-W10 (Figure 3a). This demonstrates that the structure of as-prepared C16py-W10 is not changed by the recrystallization from hot acetonitrile. The pattern of the recrystallized C16py-W10 (Figure 3b, a = 10.7588, b = 11.5068, c = 24.7480 Å, α = 99.914, β = 93.577, γ = 116.664°, V = 2662.9 Å3 [45]) was almost the same as that calculated with the single crystal X-ray analysis data (Figure 3c), indicating that the recrystallized C16py-W10 is a single phase.

Figure 3

Powder X-ray diffraction patterns of (a) as-prepared C16py-W10 and (b) recrystallized C16py-W10, and that (c) calculated with single crystal data.

2.3. Conductivity of C16py-W10

Figure 4 shows an impedance spectrum for the recrystallized C16py-W10 at 423 K. The spectrum showed a suppressed half circle in the high- and medium-frequency regions and an inclined line in the low-frequency region. The suppressed half circle consisted of two partially overlapped semicircles due to bulk and grain boundary resistances. The linear part in the low-frequency region would result from a combination of charge transfer resistance and Warburg impedance related to the diffusion of the carrier. The equivalent circuit [46,47,48] is shown in Figure 4: Rb and Cb are the resistance and capacitance of the bulk, respectively. Rgb and Cgb are the resistance and capacitance, respectively, of the grain boundary. Rct and Cdl are the charge transfer resistance and double layer capacitance, respectively. ZW is the Warburg impedance. The red line in Figure 4 represents fitted data with the equivalent circuit described above (Figure 4, inset), which successfully reproduces the measured impedance spectrum. The value of Rb obtained by the fitting was 2.25 × 104 Ω, from which the conductivity of the bulk C16py-W10 was estimated to be 4.8 × 10−6 S cm−1 considering uncertainty. The estimated value of Cb was 1.01 × 10−6 F, resulting in the time constant for the process in the bulk (Rb × Cb) of 2.27 × 10−2 s. This short time constant suggests that the bulk process occurs by electronic conduction [47], in good agreement with the fact that C16py-W10 contains no easily moving ion such as H+. Figure 5 shows the thermogravimetric (TG) curve and IR spectra of recrystallized C16py-W10. No weight loss was observed below 523 K. The weight of C16py-W10 decreased by 36% from 523 K to 723 K (Figure 5a), which was attributed to the decomposition and removal of the C16py cations. The crystallinity of C16py-W10 revealed by powder XRD decreased after the impedance spectroscopy measurements (not shown). However, IR spectra before and after the measurements at 423 K (Figure 5b) exhibited characteristic peaks for the W10 anion in the range of 400–1000 cm−1 [49], demonstrating that the molecular structure of W10 was retained after heating at 423 K. These results indicate that C16py-W10 is thermally stable below 423 K.

(a) TG curve of recrystallized C16py-W10; (b) IR spectra of recrystallized C16py-W10 before and after the impedance spectroscopy measurements at 423 K.

The conductivity of C16py-W10was much lower than the radical salts of POM containing organic donor such as bis(ethylenedithio)tetrathiafulvalene [2,3]. These radical salts have conductive layers of organic donor, which possibly leads to three-dimensional conduction. On the other hand, the conductivity of C16py-W10is considered to be two-dimensional along the inorganic layers composed of W10 and pyridinium hydrophilic heads (ab plane in the crystal). The anisotropy of the conductivity was difficult to investigate because large single crystals were not obtained. The conductivity of C16py-W10 reported here was measured for pelletized ground powder, and is considered to be averaged and overall conductivity. Although the conductivity of C16py-W10 was not so high, these results suggest that appropriate combination of POMs as electron reservoirs and surfactants with π-electrons would pave the way to another class of hybrid conductors.

3. Experimental Section3.1. Syntheses and Methods

All chemical reagents were obtained from commercial sources. C16py-W10 was synthesized according to a modified procedure of the preparation of tetrabutylammonium salt of W10 [49]. 4.0 g (12.1 mmol) of Na2WO4·2H2O was dissolved in 25 mL of water, and then the solution was boiled and acidified with 8.4 mL of boiling 3 M HCl solution (25 mmol) with vigorous stirring. After boiling for 2 min, 1.9 g (5.3 mmol) of [C5H5N(C16H33)]Cl·H2O (C16pyCl·H2O) in water/ethanol (20 mL, 1:1 (v/v)) was added to form white precipitates followed by filtration and suction to dryness. Recrystallization of the crude product from hot acetonitrile gave colorless plates. The single crystals were obtained by drying up an acetonitrile solution of C16py-W10, which was obtained by the cation exchange of sodium salt of W10 (Na-W10) [34]. Data for C16py-W10 (single crystals): Anal. Calcd for C84H152N4W10O32: C, 28.3; H, 4.3; N, 1.6%. Found: C, 28.4; H, 3.8; N, 1.6%. IR (KBr disk): 952 (m), 917 (s), 859 (m), 806 (s), 720 (w), 668 (m), 554 (w) cm−1.

IR spectra (as KBr pellet) were recorded on Jasco FT-IR 5000 and Horiba FT-710 spectrometers. Thermogravimetric and differential thermal analyses (TG-DTA) were performed on an ULVAC MTS9000 + TGD9600 system. Conductivity measurements were carried out by the alternating current (AC) impedance method in a frequency range from 5 Hz to 13 MHz using an Agilent 4192A inductance-capacitance-resistance (LCR) meter. Pelletized powder samples of recrystallized C16py-W10 (10 mm in diameter, 0.854 mm in thickness) were sandwiched with Pt electrodes, and the impedance was measured under a dry Ar atmosphere at 423 K. Bulk resistances and conductivities of C16py-W10 were estimated by the fitting of typical Nyquist plots.

3.2. X-ray Diffraction Measurements

Single crystal X-ray diffraction measurements for C16py-W10 were made on a Rigaku RAXIS RAPID imaging plate diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71075 Å). Diffraction data were collected for a platelet crystal (0.30 × 0.30 × 0.02 mm) and processed with PROCESS-AUTO [50]. The structure was solved by heavy-atom Patterson methods [51] and expanded using Fourier techniques [52]. The refinement procedure was performed by the full-matrix least-squares using SHELXL97 [53]. All calculations were performed using the CrystalStructure [54] software package. Numerical absorption correction was performed for the observed data. In the refinement procedure, all non-hydrogen atoms were refined anisotropically, and the hydrogen atoms on C atoms were located in calculated positions. Further details of the crystal structure investigation may be obtained free of charge from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336-033; or Email: deposit@ccdc.cam.ac.uk (CCDC-865932).

Powder X-ray diffraction (XRD) patterns for C16py-W10 were measured with a XRD-DSCII (Rigaku Corporation) diffractometer by using Cu Kα radiation (λ = 1.54056 Å, 50 kV-300 mA) at ambient temperature. A powder C16py-W10 sample was sieved in a 200 mesh sieve to remove large particles and to avoid preferred orientation. Diffraction data were collected in the range of 2θ = 2–30° at 0.01° point and 5 s/step. The lattice parameters were calculated using Materials Studio Softwares (Accelrys Inc.) by the peak profile fitting using the Pawley refinement [55].

4. Conclusions

Decatungstate-hexadecylpyridinium hybrid layered crystal, [C5H5N(C16H33)]4[W10O32] (C16py-W10), was successfully synthesized by a simple cation-exchange reaction. C16py-W10 was obtained as a single phase, and the crystal structure was determined by single crystal X-ray diffraction. C16py-W10 contained the stacking of W10 monolayers and C16py interdigitated bilayers. The alternating current (AC) impedance spectroscopy measurements revealed the conductivity of C16py-W10 to be 4.8 × 10−6 S cm−1 at 423 K. Although the conductivity was considerably lower than the radical salts of POM [2,3] or other layered materials [5,6], C16py-W10 shows the potential of polyoxometalate-surfactant hybrid crystals as conductive materials.

Acknowledgments

This work was supported in part by JSPS Grant-in-Aid for Scientific Research (No. 23750246), Nippon Sheet Glass Foundation, and Iketani Science and Technology Foundation.